The cancer testes (CT) antigens are a group of proteins originally defined by their normal expression in testes and their aberrant expression in melanomas and other cancers. The first CT antigens discovered were members of the MAGE family of proteins, including the MAGE-A, -B, and -C subfamilies which are encoded on the X-chromosome and which are now called Class I MAGE antigens (1, 2). Because many Class I MAGE genes are highly homologous and are co-regulated in gametogenesis and in tumors, it has been suggested that many MAGE proteins have similar or complimentary functions (3). Due to these factors and the difficulty in obtaining antibodies that differentiate between nearly identical sub-family members, most studies of MAGE gene expression rely on the use of antibodies recognizing common determinants or on the detection of mRNA, usually by reverse transcription followed by the polymerase chain reaction (RT-PCR) (3, 4). MAGE gene expression can be caused by promoter region demethylation and is widespread in malignancies, being found in 50% or more of melanomas, synovial sarcomas, and primary carcinomas of the lung, head and neck, urinary bladder, and ovaries, as in well as lesser percentages of primary breast carcinomas and myelomas (5-8).
The functions of most Class I MAGE molecules have not been determined, and it is not known whether their expression in tumors is a functionally irrelevant by-product of cellular transformation or could actually contribute to the development of malignancies (2).
KAP1, also known as TRIM28, Tif1β, or Krip1, is an ˜106 kD protein with a RING-B-box coiled-coil (RBCC) domain near its amino terminal end (9-11). Complete loss of KAP1 function in the homozygous KAP1 knockout mouse is lethal in utero in the presence of functional p53, and KAP1 is increasingly being recognized as a central molecule in gene regulation (12). Binding to the RBCC motif of KAP1 is required for function of all KRAB domain containing zinc finger transcription factors (13, 14). KAP1 appears to function as a molecular scaffold that coordinates at least four activities necessary for gene specific silencing including: 1. targeting of specific promoters through the KRAB protein zinc finger motifs; 2. promotion of histone deacetylation via the NuRD/histone deacetylase complex; 3. histone 3-K9 methylation via SETDB1; and 4. recruitment of HP1 protein (14).
Of particular interest in tumor biology is the fact that KAP1 acts as a co-repressor of p53 by binding to MDM2, RING domain ubiquitin E3 ligase and a major repressor of the p53 tumor suppressor protein, thereby suppressing p53 expression, p53 acetylation and p53 function (15). Kap1 acts as a p53 co-repressor with MDM2 by several mechanisms including 1) increasing binding of MDM2 to p53; 2) contributing to MDM2 inactivation of p53 transcription functions; 3) increasing MDM2 inhibition of p53 acetylation) and 4) promoting MDM2 mediated ubiquitination and degradation of P53 (15).
The present inventors recently reported that multiple MAGE proteins promote the viability of malignant mast cell lines, mostly by suppressing apoptosis, and other workers have shown that one MAGE molecule, MAGE-A2, binds to p53 (16, 17). U.S. Pat. Publication No. 2005/0265997 discloses a surprising discovery that inhibiting MAGE gene expression or function will inhibit tumor cell formation or tumor cell growth. In addition, sperm cell apoptosis is also induced by inhibition of MAGE gene expression or function. Preferable MAGE antagonists disclosed therein include an anti-MAGE antibody, an antisense molecule, an siRNA molecule, a molecule for forming a triplex nucleic acid molecule with a MAGE encoding polynucleotide, or a small molecule inhibitor of MAGE function.
However, there remains a need for methods and compositions that inhibit the function of MAGE proteins by targeting specific downstream effectors on which MAGE proteins act.